Electroflux slags and methods of electroflux remelting

ABSTRACT

A METHOD AND A SLAG ARE PROVIDED FOR REMELTING SULFURIZED TOOL STEELS SO AS TO RETAIN CONTROL OF THE SULFUR. THE METHOD OF THE INVENTION INCLUDES MELTING A SULFURIZED TOOL STEEL, FORMING THE STEEL INTO AN ELECTRODE FOR ELECTROFLUX REMELTING, THE ELECTRODE IN AN ELECTROFLUX REMELT PRACTICE UNDER A SLAG HAVING THE COMPOSITION CAF2 ABOUT 35% TO ABOUT 60%, AL2O3 ABOUT 20% TO ABOUT 40%, SIO2 ABOUT 10% TO ABOUT 30% AND CASO4 ABOUT 3% TO 105 AND FINALLY SOLIDIFYING THE REMELTED SULFURIZED TOOL STEEL AS AN INGOT.

United States Patent 3,723,094 ELECTROFLUX SLAGS AND METHODS OF ELECTROFLUX REMELTING Rene Schlatter, Derry Township, Westmoreland County, Pa., assignor to Latrobe Steel Company No Drawing. Filed Sept. 1, 1971, Ser. No. 177,177 Int. Cl. C21c /52, 7/02 US. Cl. 75-12 6 Claims ABSTRACT OF THE DISCLOSURE A method and a slag are provided for remelting sulfurized tool steels so as to retain control of the sulfur. The method of the invention includes melting a sulfurized tool steel, forming the steel into an electrode for electroflux remelting, remelting the electrode in an electroflux remelt practice under a slag having the composition CaF about 35% to about 60%, A1 0 about 20% to about 40%, SiO about to about 30% and CaSO, about 3% to 10% and finally solidifying the remelted sulfurized tool steel as an ingot.

This invention relates to electroflux slags and methods of electroflux remelting and particularly to a slag and method of electroflux remelting for controlled retention of sulfur in sulfurized tool steels, such as high-speed, hot and cold work die steels.

The use of sulfur as a modifier of special tool steels is well known and these sulfurized steels have become an important class of steel grades in the specialty steel industry. Unfortunately, sulfur control in electroflux remeltingof the sulfurized tool steel grades has proven to be very difiicult. Ordinary electroflux remelting has been completely unreliable with the sulfur content of the steel varying drastically and uncontrollably from the bottom to the top of an ingot. Various attempts have been made to solve this problem but without success. For example, the addition of iron sulfide to a binary CaF -Al O type slag with relatively low sulfur capacity resulted in excessive compositional variations varying from 0.16% sulfur at tthe ingot bottom to 0.08% sulfur at the top of the same ingot. Such variations are, of course, intolerable. Similarly, the use of small amounts of Cas along with FeS produce somewhat better sulfiur retention values but the CaS addition made the slag extremely sensitive to humidity and the resultant reliability of sulfur control was unsatisfactory. Both of these practices should, in theory, have produced an improved result but in fact did not.

I have discovered a method and a slag composition which eliminates the problem of lack of sulfur control in tool steels. I have found that the addition of a minor amount of calcium sulfate (CaSO will stabilize the sulfur in a sulfurized steel, and make it possible to control the sulfur content of these steels in electroflux remelt practices within reasonable limits. I have found that reversed polarity D.C. power is as elfective as straight polarity D.C. power in the control of sulfur retention in tool steels. A.C. power in combination with the slag composition of this invention also provides good sulfur control. The flux of this invention overcomes the problem of desulfurizing effect common to normally used slags when using reverse D.C. or A.C. power. Thus, the flux composition of this invention is of great benefit in A.C. and reverse polarity D.C. melting which are the two modes most commonly used in electro slag remelting.

In the practice of my invention I preferably form an electrode of the desired composition of sulfurized tool steel, remelt the electrode in an electroflux remelt furnace using a flux basically consisting of CaF A1 0 and SiO with a minor portion of CaSO and a reverse polarity D.C. or A.C. electrical power system. Preferably the flux has a composition in the range:

Percent, about caF, 35-60 41 0, 20-40 $10 10-30 caso, 3-10 The preferred flux composition is:

Percent, about CaF 40 A1 0 30 SiO 25 C3804 5 A high carbon high-speed steel of the grade M25 and having the composition:

Percent C .95 Mn .37 Si .34 Cr 4.03 V 1.97 Mo 4.87 W 6.37 S .120

was melted in an electric furnace and formed into electrodes. The electrodes were then placed in a hollow copper mold and remelted under a slag having the composition:

Percent CaFg A1 0 30 SiO 20 CaSO, 5

The analysis of the remelted steel ingot showed it 613's essentially uniform from top to bottom and to have the composition:

Percent C .97 Mn .34 Si .33 Cr 4.06 V 1.86 Mo 476 W 6.35 S (at ingot top) .115 S (at ingot bottom) .110

These results show that the sulfur retention has been stabilized at a high level for the entire ingot length.

EXAMPLE II A hot-work die steel of the grade H-13S having the composition:

was melted in an electric furnace and formed into electrodes. The electrodes were then placed in a copper mold and electroflux remelted under a slag having the composition:

Percent CaF A1 30 SiO 25 CaSO 5 The analysis of the remelted steel ingot showed it to be essentially uniform in sulfur content from top to bottom and to have the composition:

Percent C .38 Mn .80 Si .91

Again the results show the sulfur content to be stabilized and uniform, a result unobtainable with conventional electroflux remelt practices.

EXAMPLE III A sulfurized cold-work die steel of the grade D-2S having the composition:

Percent Mn .49 Si .30 Cr 11.58

was melted conventionally in an electric furnace and cast into electrodes. The electrodes were then electroflux remelted under a slag having the composition:

Percent CaF2 50 A1 0 5 25 SiO 20 C3804 5 The analysis of the remelted steel ingot showed a uniform sulfur content from bottom to top and a composition as follows:

Percent C 1.52 Mn .44

The results indicate good sulfur control and uniform sulfur distribution in the ingot- In the foregoing specification 1 have set out certain preferred embodiments and practices of my invention, however, it will be understood that this invention may be otherwise practiced within the scope of the following claims.

I claim:

1. The method of retention of sulfur in electroflux remelting of special tool steels comprising the steps of:

(a) melting a sulfurized tool steel composition in a conventional furnace,

(b) forming the molten sulfurized tool steel into electrodes for electroflux remelting,

(c) melting the sulfurized tool steel electrodes in an electroflux practice under a slag having the composition CaF about 35% to about 60%, A1 0 about 20% to about 40%, SiO about 10% to about 30% and 0.150 about 3% to about 10%, and

-(d) solidifying the remelted sulfurized tool steel as a final ingot.

2. A method as claimed in claim 1 wherein the slag composition is CaF about 40%, A1 0 about 30%, SiO about 25% and the balance CaSO 3. A method as claimed in claim 1 wherein the electrodes are remelted under reversed polarity electric power.

4. A method as claimed in claim 2 wherein the electrodes are remelted under reversed polarity electric power.

5. A slag for remelting sulfurized high-speed steel comprising CaF about 35% to about 60%, A1 0 about 20% to about 40%, Si0 about 10% to about 30% and CaSO; about 3% to about 10%.

6. A slag as claimed in claim 5 comprising CaF' about 40%, A1 0 about 30%, SiO, about 25 and the balance CaSO References Cited UNITED STATES PATENTS 2,540,173 2/1951 Olivo -58 2,694,023 11/-1954 Hopkins 148-26 2,747,990 5/1956 Morrogh 75-53 2,848,321 8/ 1958 Molesworth 75-94 2,979,393 4/1961 Heuer 75-51 3,340,103 9/1961 Ballass 148-26 3,551,137 12/1970 Bhat 148-26 3,155,492 11/1964 Udy 75-30 L. DEWAYNE RUTLEDGE, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 

